The crystal structures and optical properties of (E)-2-methoxy-6-(pyridin-3-yliminomethyl)phenol (1), a N-salicylidene-3-aminopyridine, and its 2:1 dimorphic cocrystals with fumaric acid (2 and 3) and succinic acid (4 and 5) were studied at solid state in order to understand the effects of cocrystallization and molecular packing on their photo-and thermochromic behavior. New criteria for prediction of the chromic properties, based on the rationalization of the intermolecular interaction in the crystal, are also proposed. These results confirm once more the influence of the molecular packing on the chromic properties of solids and extend it to polymorphic and isomorphic cocrystals.

N-Salicilideneanilines are among the most studied thermo- and photochromic systems in the solid state. Although thermochromism is a general property of crystalline N-salicilideneanilines, photochromism is known in a limited number of cases. As a method for the construction of thermo- and photo-responsive molecular architectures, the co-crystallisation of 1,2,4,5-tetrafluoro-3,6-diiodobenzene (I2F4) with three selected imines of o-vanillin, named 1, 2 and 3, obtained through a condensation reaction with 3-aminopyridine, 4-bromoaniline and 4-iodoaniline, respectively, is reported herein. All crystals and co-crystals have been characterised by means of solid-state complementary techniques (X-ray diffraction, solid-state NMR spectroscopy, absorption and emission spectroscopy). The role of halogen bonding and crystal packing in the optical and chromic properties of all solid materials is discussed. All solids exhibit thermochromic behaviour, and three of them (2, 2 ⋅I2F4 and 3 ⋅I2F4) are also photochromic. Imine derivative 3 crystallises in two different polymorphic forms (3 A and 3 B) and a solvate (3 ). The bromo and iodo derivatives, 2 and 3 B, are isomorphous and form isomorphous co-crystals with I2F4, but behave differently when exposed to UV light because only crystalline 2 is photochromic. Interestingly, the replacement of bromine with iodine seems to turn off the photochromism because crystalline 3 A and 3 , and even the 2 3 solid solution, do not manifest photochromic behaviour.

Methylcellulose (MC), a water-soluble polymer derived from cellulose, was investigated as a possible temporary substrate having thermo-responsive properties favorable for cell culturing. MC-based hydrogels were prepared by a dispersion technique, mixing MC powder (2, 4, 6, 8, 10, 12 % w/v) with selected salts (sodium sulphate, Na2SO4), sodium phosphate, calcium chloride, or phosphate buffered saline, to evaluate the influence of different compositions on the thermo-responsive behavior. The inversion test was used to determine the gelation temperatures of the different hydrogel compositions; thermo-mechanical properties and thermo-reversibility of the MC hydrogels were investigated by rheological analysis. Gelation temperatures and rheological behavior depended on the MC concentration and type and concentration of salt used in hydrogel preparation. In vitro cytotoxicity tests, performed using L929 mouse fibroblasts, showed no toxic release from all the tested hydrogels. Among the investigated compositions, the hydrogel composed of 8 % w/v MC with 0.05 M Na2SO4 had a thermo-reversibility temperature at 37 °C. For that reason, this formulation was thus considered to verify the possibility of inducing in vitro spontaneous detachment of cells previously seeded on the hydrogel surface. A continuous cell layer (cell sheet) was allowed to grow and then detached from the hydrogel surface without the use of enzymes, thanks to the thermo-responsive behavior of the MC hydrogel. Immunofluorescence observation confirmed that the detached cell sheet was composed of closely interacting cells.

Almost 200 years ago, benzamide was reported as polymorphic with two of its forms (II and III) found to be difficult to crystallise. In a recent study, it was shown that benzamide form I can easily convert into benzamide form III using mechanochemistry in the presence of nicotinamide. Here we show, experimentally and computationally, that this transformation is the result of a thermodynamic switch between these two polymorphic forms driven by the formation of solid solutions with small amounts of nicotinamide. The presence of nicotinamide in the crystallisation environment promotes the robust and exclusive crystallisation of the elusive form III. These results represent a promising route to the synthesis and utilisation of elusive polymorphs of pharmaceutical interest.

The geometrical structures of salicylideneaniline (anil) molecular switches in the solid state have been determined using periodic structure calculations and a variety of density functional theory (DFT) exchange-correlation (XC) functionals, of which several have been tuned for the solid state. The first target was on predicting the unit cell and intramolecular geometrical parameters for three anil derivatives, i.e., the (E)-2-methoxy-6-(pyridine-3-yliminomethyl)phenol (PYV3) and N-(5chloro-2-hydroxybenzylidene)-aniline (HC) crystals, where the enol (E) form is dominant in the crystalline state at low temperature (∼100 K), and the N-(5-chloro-2-hydroxybenzylidene)-hydroxyaniline (POC) crystal, which is mostly composed of the keto (K) form. The best performance for the unit cell parameters, in comparison with single-crystal X-ray diffraction (XRD) data, is achieved with XC functionals developed for the solid state (PBEsol and PBEsol0) as well as with ωB97X. On the other hand, the differences between the functionals are much smaller when considering the bond lengths and the valence angles so that the deviations with respect to XRD data in the bond length alternations of the key O−CC−CN−C (or OC−CC−N− C) π-conjugated segment are smaller than 0.02 Å for PBEsol0 and ωB97X. Similar trends are observed for the two polymorphic cocrystals of PYV3 with fumaric or succinic acid. The second target was the characterization of the variations of energy and structural parameters when switching between the enol and keto forms. All XC functionals predict that PYV3 presents a larger ΔE KE value than HC, and as expected, both are larger than for POC. Still, only hybrid functionals correctly predict which form is the most stable in the crystalline state. Then, the bond length changes in the O−CC−CN−C (or OC−CC−N−C) πconjugated segment that occur upon enol to keto transformation are similarly predicted by all functionals and are consistent with the reversal of the single/double bonds pattern.

In N-salicylideneaniline derivatives, photochromism occurs by a two-step isomerization mechanism. Photochromism has been reported to be closely related to the molecular conformation of the N-salicylideneanilines (described by the dihedral angle Φ) and to the free available space (V free ) in the crystal. In this contribution, we focus on cocrystals of isomeric Nsalicylideneaminopyridines with perfluorinated halogen bond donors as coformers. The advantage of working with isomers is that they have a similar (if not equal) molecular volume. This aspect means that the evaluation and comparison of the free available spaces within cocrystals is not affected by the differences in the molecular volumes of their constituents. Cocrystals were synthesized by a one-pot procedure, which allowed concomitant formation of covalent (imine) and noncovalent (halogen) bonds. Each cocrystal was characterized by X-ray diffraction, and its photochromism was assessed by UV−vis diffuse reflectance. Our results suggest that the Φ/V free model grossly oversimplifies the complexity of the dynamic phenomenon behind the photoisomerization of N-salicylideneanilines. For the above-mentioned reasons, careful examination of intermolecular interactions and crystals packing (localization of voids, evaluation of intermolecular interactions and stacking modes) should be taken into account and used to supplement the current proposed model based on Φ and V free .

A number of salophen ligands and their Zn, Ni, and Pd complexes were synthesized by an efficient one-pot mechanosynthesis protocol. The reaction products were characterized by means of complementary solid-state techniques, i.e., powder X-ray diffraction, single-crystal X-ray diffraction, and solid-state NMR spectroscopy. Four new crystal structures of metal salophen complexes as DMSO solvates are here reported. The described simple and relatively fast (about 1 h for all derivatives) procedure is a good alternative to classical methods performed in organic solvents.

In the context of solid-state polymorphism, a detailed crystallographic and theoretical investigation has been carried out on a dimorphic phenylthiazole-derived compound (4-phenylthiazole-2-thiol, labeled here as compound 1), considered here as a potential lead compound, for which we discovered original polymorphic behavior. A detailed crystallographic study and calorimetric investigation permit us to assess the monotropic relationship between polymorphic forms, and ab initio quantum methods, combined with Monte Carlo polymorphic prediction, allow rationalization of the thiol-thione tautomeric preferential character of the compound. The experimental and theoretical conclusions prove once more that in an early process of lead compound optimization, a combination of solid-state analysis and phase prediction with modeling methods can be a fast and efficient tool to select/disable a particular compound, in addition to well-known potential alert flags such as adsorption–distribution–metabolism–excretion/toxicology pharmacokinetic markers.